This invention relates to a colour picture tube and a method for adjusting a colour picture tube. In order to provide a good colour purity in a colour picture tube, three electron beams emitted from the electron gun assembly have to correctly impinge on the centers of the respective red, green and blue phosphor strips or dots formed on the face plate of the tube. Adjustment for colour purity is unnecessary, if components of a colour picture tube are manufactured in a sufficient preciseness and assembled together in a sufficient precise fashion. But certain practical limitations are imposed on preciseness of mechanical components and tolerance errors are unavoidably involved when they are assembled, so that any colour picture tube calls for adjustment for colour purity. An error caused particularly when a shadow mask is mounted inside of the skirt section of the face plate often necessitates such a later adjustment for colour purity. Specifically, in a colour picture tube assembling process, some internal or external stress may be often applied to a shadow mask, so that dot apertures or slits in the shadow mask will be shifted out of alignment with the corresponding dots or phosphor strips on the face plate, and/or so that the shadow mask itself will be deformed. Such shift of the apertures or slits of the shadow mask and deformation of the shadow mask are as small as the order of 10 to 30 microns, but when the related components are affected by terrestrial magnetism, such errors are enlarged to a degree enough to render colour purity of the tube poor.
Conventionally, fine adjustment of a mounting position of the deflection yoke and adjustment of the purity magnet are made to correct a poor colour purity so that good colour purity can be obtained by the assembled colour picture tube.
As well known in the art, the deflection yoke requires a higher order of technique in its positional adjustment. Moreover, it is heavy, 500 to 1700 g, and after adjustment it is gradually displaced with its own weight. Further, not only the deflection yoke, but colour purity magnet and convergence yoke have to be adjusted in the operation of colour purity adjustment, so that the adjustment operation disadvantageously requires long time. Another problem has frequently been experienced in the conventional method of colour purity adjustment that while the intended areas of the shadow mask can be fully adjusted, the other areas remain not fully adjusted, i.e., that the entire area of the shadow mask can not be correctly adjusted.